A medical material to be contacted with a body fluid, such as an artificial blood vessel, a catheter, a blood bag and a blood treatment apparatus, has been required to have high anti-thrombotic properties. Examples of the blood treatment apparatus include an artificial kidney, an artificial liver, an artificial lung, a blood component adsorbent device and a plasma separator. In the present invention, a blood treatment apparatus is synonymous with a blood purification apparatus, and a hollow fiber membrane module refers to a hollow fiber membrane-type blood treatment apparatus.
For example, in a hollow fiber membrane for use in an artificial kidney (of which the schematic cross sectional views are shown in FIGS. 1 and 2), the deposition of a protein or the deposition/activation of platelets can cause the coagulation of blood. When a protein or the like is deposited onto a membrane, even if led to the coagulation of blood, pores in the membrane are blocked out and become small, resulted in the deterioration in the performance. When the performance of the membrane is altered rapidly within a short time, there is a concern about the increase in burden on a living body.
For the purpose of solving these problems, it has been attempted to hydrophylize a hollow fiber membrane and various studies have been made for this purpose. For example, a method is disclosed, in which polyvinylpyrrolidone, which is a hydrophilic polymer, is mixed with polysulfone in the stage of a membrane forming stock solution and the resulting mixture is molded to thereby impart hydrophilicity to a membrane and protect the membrane from stains (Patent Document 1). However, merely the addition of a hydrophilic component to a membrane forming stock solution cannot achieve a satisfactory deposition-preventing effect. Then, various improvements have been attempted. For example, a method in which a vinylpyrrolidone-type polymer as well as a polyglycol are added to a membrane forming stock solution to thereby increase the amount of the vinylpyrrolidone-type polymer present on the inner surface of a membrane (Patent Document 2) and a method in which a vinyl acetate group is provided on the surface of a membrane (Patent Document 3) are disclosed. In addition, a method in which a hydrophilic monomer is graft-polymerized onto the surface of a material (Non-Patent Document 1) is also disclosed. However, as a result of the extensive studies made by the present inventors, it is found that these methods are insufficient for developing anti-thrombotic properties. This is probably because attention is focused only on a hydrophilic polymer on the surface, adsorbed water in the polymer is not taken into consideration, and the physical configuration of the surface of a membrane is insufficient.
Further, in the case of an artificial kidney, after the completion of a blood dialytic therapy, a blood returning procedure in which a saline solution is allowed to pass through the artificial kidney and blood remaining in the artificial kidney and the blood circuit is returned into the body of a dialysis patient is carried out. However, blood that cannot be returned into the body is sometimes still remained in the artificial kidney, which is a phenomenon called “residual blood”. The residual blood often occurs in an artificial kidney having poor anti-thrombotic properties, can cause anemia in a dialysis patient, and therefore should be avoided. Heretofore, various improvement methods have been proposed. As an invention for solving the problem of residual blood induced by the accumulation of blood in a zone that is the farthest from the center of axis of a main body case 10 (also referred to as “an outer peripheral part”, hereinbelow) in header inner spaces 27 and 28 in a blood treatment apparatus 1 as shown in FIG. 2, for example, a method is proposed in which the clearance C between the outer peripheral surface of a hollow fiber membrane bundle 40 and the inner peripheral surface of each of headers 21 and 23 in each of partitioning wall edge faces 31 and 33 is reduced to thereby reduce the accumulation of blood (Patent Documents 4 and 5).
However, as a result of the repeated experiments made by the present inventors, it is found that the occurrence of residual blood is often observed even in an artificial kidney having a sufficiently small clearance C and therefore the above-mentioned inventions are insufficient for solving the problem of residual blood.